1. Field of the Invention
The present invention relates to a power supply of an apparatus that has a normal operating mode and a standby mode in which energy is saved.
2. Description of Related Art
FIG. 13 illustrates an arrangement of a power supply apparatus for obtaining a conventionally known regulated DC power supply.
An appliance on which the power supply apparatus is mounted is arranged to supply two levels of voltage: a first direct current for a driving section that includes, for example, a motor and a solenoid and requires a relatively high voltage to operate, and a second direct current for a control section that requires a low voltage to operate a CPU, an ASIC and the like.
In addition, since the driving section is not operated in the standby mode, i.e. when the apparatus is in the energy-saving state, a load in the driving section need not be supplied with voltage. The apparatus, therefore, is arranged to block the voltage supplied to the load in the driving section by means of a load switch (not illustrated) or the like in the standby mode. In FIG. 13, a first DC/DC converter A supplies a power supply voltage for the driving section and a second DC/DC converter B supplies a power supply voltage for the control section.
With reference to FIG. 13, detailed description will now be made. The components illustrated in FIG. 13 are arranged as described below.
The apparatus includes a commercial alternating voltage source 700, a rectifier 702, a smoothing capacitor 703, a start-up resistor 705, a switching element 707, a power supply control IC 710, a transformer 711, a diode 712 and a capacitor 713. The apparatus further includes a secondary rectifying diode 720, a secondary smoothing capacitor 721, resistors 722, 723 and 724, and a shunt regulator 750. The apparatus further includes an LED-side photocoupler 714-b, a capacitor 728, and an FET 732 serving as switching means for the DC/DC converter that generates the second direct current from the first direct current. The apparatus further includes a gate resistor 734, FET drive transistors 733 and 735, a control IC 738 for controlling the second DC/DC converter, an inductor 739, a diode 740, a capacitor 741 and resistors 742 and 743. The apparatus further includes a load 731 (in the driving section) for the first direct current, and a CPU 746 (in the control section) serving as a load for the second direct current.
First, the operation of the first DC/DC converter apparatus is described below.
When an alternating current is applied from the commercial alternating voltage source 700, the capacitor 703 is charged with a voltage rectified by the rectifier 702. The rectifier 702 and the capacitor 703 function as a rectification smoothing circuit for rectifying and smoothing the alternating current from the alternating voltage source. As the voltage across the capacitor 703 increases, power is supplied to the power supply control IC 710 through the start-up resistor 705, and the power supply control IC 710 then turns on the FET 707. Once the FET 707 is turned on, a current flows through a primary winding Np of the transformer 711, and a voltage applied to the Np winding of the transformer 711 causes a voltage to appear on windings Ns and Nb. The voltage appearing on the winding Nb is blocked by the diode 712 to prevent a current from flowing, and the voltage on the winding Ns is similarly blocked by the diode 720 to prevent a current from flowing. The FET 707 is turned off after a predetermined period defined by an internal circuit of the power supply control IC 710. This causes the voltage to increase on the winding Np on the drain side of the FET 707. A current flows through the winding Ns via the diode 720 in the direction so as to charge the capacitor 721; as the capacitor 721 is charged, the voltage across the capacitor 721 increases. After a predetermined period defined by the internal circuit of the power supply control IC 710, the FET 707 is turned on and a current is again supplied to the transformer 711 from the capacitor 703. When the power supply control IC 710 turns off the FET 707 after a predetermined period, the capacitor 721 is again charged through the diode 720 with the voltage on the winding Ns. The voltage across the capacitor 721 is divided by the resistors 723 and 724, and a voltage across the resistor 724 is applied to a control terminal of the shunt regulator 750. A cathode current of the shunt regulator 750 is sent the power supply control IC 710 through the photocoupler 714-b. 
A reference voltage in the shunt regulator 750 is compared to the voltage across the resistor 724 divided by the resistors 723 and 724, and if the voltage across the resistor 724 is higher than the reference voltage, the apparatus operates to reduce ON width or ON duty of the FET 707 so as to reduce the output voltage. If the voltage across the resistor 724 is lower than the reference voltage in the shunt regulator 750, the apparatus performs a feedback operation such that the ON time or ON duty of the FET 707 is increased so as to increase the output voltage.
Next, the operation of the second DC/DC converter apparatus is described below.
The second DC/DC converter generates the second direct current from the output voltage of the first DC/DC converter. In the normal mode, the second DC/DC converter control IC 738 intermittently drives the FET 732 through the transistors 733 and 735, and the resistor 734. The resistors 742 and 743 divide the output voltage of the second DC/DC converter, and a voltage across the resistor 743 is input to the second DC/DC converter control IC 738. The second DC/DC converter control IC 738 has an internal reference voltage Vref2, and controls ON duty of the FET 732 such that the voltage across the resistor 743 is equal to the Vref2 to generate the stabilized second direct current. In this way, the apparatus is provided with the load switch on the output side, where the power supply voltage of the driving section is output, in order to reduce power in the standby mode, and is arranged to turn off the load switch in the standby mode by means of a control circuit, such as a CPU and an ASIC, operated through a control section power supply.
With the arrangement illustrated in FIG. 13, however, it is inevitable that, as the load is reduced, the efficiency of the DC/DC converter is also reduced. To solve the problem of the reduced efficiency, for example, Japanese Patent Application Laid-Open No. 2000-278946 discloses an arrangement that, in an RCC-type switching power supply apparatus, drops the output voltage in the standby mode and supplies to the load the output voltage raised to a desired value by a subsequent DC/DC converter. In the arrangement of Japanese Patent Application Laid-Open No. 2000-278946, the output voltage is reduced in an RCC-type converter apparatus to reduce a ringing voltage on an auxiliary winding below a threshold of a switching element while the switching element is turned off. In this way, a flyback voltage is prevented from turning on the switching element and OFF time of the main switching element is extended to reduce the oscillation frequency. As a result, a switching loss is reduced and circuit efficiency is improved.
While the appliance is in the standby mode, however, a load current in the control section is also reduced. Accordingly, the efficiency of the second DC/DC converter described above is also reduced in the standby mode of the appliance. In the arrangement of Japanese Patent Application Laid-Open No. 2000-278946, although improvement of the reduced efficiency of the first DC/DC converter due to the reduced load current is addressed, improvement of the reduced efficiency of the second DC/DC converter is not addressed.